The harmful effects on the human body of certain radioactive and non-radioactive gaseous substances has long been known. Moreover, many of these substances, existing underground, have been found to be present, not only in environments in which they are handled, but also in ordinary everyday living environments. All of which has led to a demand for more stringent monitoring of such substances, for the purpose of both research and prevention.
At international level, regulations governing the prevention and control of exposure to harmful radioactive and non-radioactive substances are currently in force, pursuant to regulations and directives issued by major national and international authorities (e.g. the International Committee for Radiation Protection (ICRP), the Environment Protection Agency (EPA) in America, and EU Directives covering all member countries).
In this connection, regulations recently issued in Italy governing radiation protection (Acts 241/2000 and 257/2001, pursuant to respective EU Directives) call for compulsory control of exposure to radon in working environments, and also provide for stricter monitoring and protocols.
The most commonly used measuring method is based on passive integrating trace detectors, which monitor the average radon concentration in inhaled air. This quantity is directly related to exposure, which is defined as the product of average concentration and the length of stay in the environment in which the detectors are installed. Using appropriate conversion coefficients, internal exposure of the respiratory system can then be determined.
Known radon detecting devices normally comprise a measuring cell of a few tens cc in volume; a filtering device, which allows radon into the measuring cell and retains the particulate present in the air; and a track detector housed inside the measuring cell, and which registers the alpha particles emitted by the radon and its decay products. A characteristic common to all these devices is that of having a constant-volume measuring cell.
Detecting devices of the above type are installed in the environment for an appropriate length of time, during which, radon penetrates by diffusion through the filtering device, and is detected by the detector as described above.
A major drawback of devices of this type lies in the measuring cell being brought into equilibrium with the outside environment as regards radon concentration (so-called initial transient) and being emptied of radon (so-called tail effect) within a given time period, which varies depending on the characteristics of the filtering device.
Another drawback lies in the detector not being protected, and so being subjected to continuous radiation, i.e. with no possibility of limiting its detection action to predetermined times and/or locations.
Both these drawbacks combined obviously have a negative effect on monitoring accuracy, by the detector being irradiated both before and after the correct monitoring time, thus supplying false radon concentration and exposure values.
That is, errors may be produced by irradiation caused by radon decay during the initial transient and during the tail effect when monitoring or calibrating in controlled atmospheres, or by decay of atmospheric radon entering the device during transport and storage.
To at least partly eliminate the above drawbacks, devices have been devised comprising mechanical shutters for protecting the detector.
Such solutions are often complex, are not particularly effective, and at best only act as safeguards during transport and storage of the device, leaving the initial transient and tail effect problems unsolved.